Basic aggregate-address:Why network not included on advertisement?

Answered Question
Mar 4th, 2010

Hi , can you help me understand what is wrong with this:

Basic topology R1-----R2(AS 100)------------R4(AS 200)

I shutdown R3 and removed all route-maps and prefix-lists for simplicity.

On R1 I have IP 10.177.207.1/24 and 10.177.206.1/24.

On R2 I do 'aggregate-address 10.177.206.0 255.255.254.0'. I also do 'network 10.177.206.0 255.255.255.0' since I understand I must advertise at least one network as part of the aggregate entry.

Then on R4, I was expecting to reach 10.177.207.1. However, that is not working.

Question:

If I have the aggregate-address command inlcuding 10.177.207.0/24, why is that R4 is not learning 10.177.207.0/24 then?

Router configurations are attached.

R2#
Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 1 subnets
B       4.4.4.4 [20/0] via 10.2.47.193, 00:01:23
     10.0.0.0/8 is variably subnetted, 7 subnets, 5 masks
B       10.2.0.0/16 [200/0] via 0.0.0.0, 00:01:22, Null0
O       10.2.80.0/24 [110/2] via 10.177.206.1, 00:13:00, FastEthernet2/0
C       10.2.60.104/30 is directly connected, FastEthernet1/0
O       10.177.207.0/24 [110/2] via 10.177.206.1, 00:13:00, FastEthernet2/0
C       10.177.206.0/24 is directly connected, FastEthernet2/0
C       10.2.255.32/32 is directly connected, Loopback0
C       10.2.47.192/28 is directly connected, FastEthernet0/0
     192.168.1.0/32 is subnetted, 1 subnets
O       192.168.1.1 [110/2] via 10.177.206.1, 00:13:00, FastEthernet2/0
R2#

R4#
Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 1 subnets
C       4.4.4.4 is directly connected, Loopback0
     10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
B       10.2.0.0/16 [20/0] via 10.2.47.204, 00:03:36
C       10.2.55.0/24 is directly connected, FastEthernet0/0
B       10.2.80.0/24 [20/2] via 10.2.47.204, 00:03:36
B       10.177.206.0/24 [20/0] via 10.2.47.204, 00:03:36  <== it is not learning the aggregate 10.177.206.0/23.
C       10.2.47.192/28 is directly connected, FastEthernet1/0
R4#
R4#
R4#show ip route 10.177.207.1
% Subnet not in table
R4#

Correct Answer by Jon Marshall about 6 years 11 months ago

Marlon

Your aggregate address statement on R2 is wrong -

you have

aggregate-address 10.177.206.0 255.255.255.252

it should be

aggregate-address 10.177.206.0 255.255.252.0   ie. the subnet mask needs modifying

Jon

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Correct Answer
Jon Marshall Thu, 03/04/2010 - 10:13

Marlon

Your aggregate address statement on R2 is wrong -

you have

aggregate-address 10.177.206.0 255.255.255.252

it should be

aggregate-address 10.177.206.0 255.255.252.0   ie. the subnet mask needs modifying

Jon

news2010a Thu, 03/04/2010 - 10:26

Ooops :-) that did it!

aggregate-address 10.177.206.0 255.255.254.0

Jon Marshall Thu, 03/04/2010 - 10:55

Marlon

Need to clarify something i my have misled you on (not intentionally )

In this thread you said -

On R2 I do 'aggregate-address 10.177.206.0 255.255.254.0'. I also do 'network 10.177.206.0 255.255.255.0' since I understand I must advertise at least one network as part of the aggregate entry.

That is not strictly true and i know it was me that led you to believe this.

For the aggregate address to be advertised at least one of the subnets making up the aggregate address must be in the BGP table. Note that is the BGP table, viewed by doing a "sh ip bgp", and not the routing table. So how do you get routes into the BGP table -

1) by receiving routes from BGP peers

2) by using the network command under the "router bgp

3) by redistributing another routing protocol into BGP

So as long as when you do a "sh ip bgp" you see one of the subnets of the aggregate address you do not then need to use a network statement.

SIncere apologies for the misleading information, my fault for not concentrating.

Jon

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